Immunofluorescence using a monoclonal antibody directed against the L3T4 receptor has revealed a pattern of neuronal labeling in cultures of fetal mouse hippocampus. Nerve growth factor appears to have neither growth-promoting nor growth-inhibiting effects on neurons from the ventral horn of the fetal mouse spinal cord. Afferents of DRG neurons converging on spinal cord neurons of mice in vitro form more and stronger synapses when exposed to a phasic pattern of electrical stimulation during development. Synaptic efficacy was diminished in competing afferents which were not provided with electrical stimulation, but the number of axonal inputs was not altered. Activity-dependent synaptogenesis and modification of synaptic boutons, while strengthening some connections, may not progress to elimination of weak convergent afferents.